The only valid HEPSPEC06 result (for current GridPP use) is the SL5 (64 bit OS) but 32 bit compiler result but for interest we ran also with 64 bit compiler switches.
Then we installed SL6 and re-ran both 32 and 64 bit compiler options.
The results are on the GridPP wiki, but the most notable thing is the performance boost you can get going from 32bit on SL5 to 64 bit on SL6.
The boost is nearly 25% which could mean a lot to the experiments and the sites productivity if they can be persuaded to migrate sooner rather than later.
https://www.gridpp.ac.uk/wiki/HEPSPEC06#UKI-SOUTHGRID-OX
